Since lead-acid batteries were invented by Plante, a French Engineer in the middle of the nineteenth century, lead-acid batteries have been widely used as cheap and reliable secondary batteries in automobile, electric vehicle, energy storage, and other fields. According to the latest statistics, though lead-acid batteries confront keen competition from Li-ion batteries and Ni—H batteries in the secondary battery market, lead-acid batteries have been taking 65.2% of market share, equivalent to USD39.294 billion in the secondary battery market owing to their unique safety performance and high cost-performance ratio. According to the statistical data from the International Lead and Zinc batteries Study Group, in 2012, the lead consumption in the world was 10.62 million tons, about 82% of which was used for producing lead-acid batteries. According to the statistical data from China Nonferrous Metals Industry Association, in 2012, the total consumption of lead in China was 4.646 million tons, in which 3.3 million tons were used to produce lead-acid batteries. In year 2012, as the last original lead mine smelting enterprise was closed and only 15 secondary lead smelting enterprises were kept in USA, facing the huge lead-acid battery demand, it is believed that waste and worn lead-acid batteries will be a major mineral asset in the society and will be increasingly used as the principal raw material for lead smelting.
The existing lead recovery process essentially belongs to pyrometallurgy of lead. Usually, the lead in lead-acid batteries mainly includes lead alloy in the plate grids and conducting tabs and lead paste in the negative electrode and positive electrode. Since lead paste contains Pb (10-15 wt %), PbO (10-20 wt %), PbO2 (25-35 wt %), and PbSO4 (30-45 wt %), lead recovery from lead paste becomes the focus in the entire lead recovery process. Automatic lead-acid battery crushing and separation equipment from Engitee (an Italia company) in combination with a device for pre-desulphurization with sodium carbonate and a device for absorbing sulfur dioxide in tail gas with sodium carbonate solution are utilized by modern pyrometallurgy enterprises, and sulfur dioxide emission in the smelting process are significantly reduced. Representative enterprises include Henan Yuguang Gold & Lead Co., Ltd., Hubei Jinyang Metallurgical Incorporated Co., Ltd., and Zhejiang Tianneng Group, etc. Though modern pyrometallurgy enables large-scale continuous production and is matured in technology, it involves pyrolytic smelting of lead-containing materials at 1100-1300° C., which not only brings a problem of high energy consumption, but also produces lead vapor resulted from inevitable volatilization at a high temperature, lead-containing dust in particle size equal to or smaller than PM2.5 and lead-containing waste residue and flue ash in the smelting process; consequently, the lead recovery rate is usually 95-97%.
To overcome the drawback of high energy consumption and lead emission in pyrometallurgy of lead, hydrometallurgy of lead is employed and regarded as a cleaner next-generation lead recovery process. Existing secondary lead hydrometallurgy processes, represented by hydrofluosilicic acid lead electrolysis, are unacceptable in industrial production owing to their high processing cost incurred by complex lead paste treatment process, high power consumption as high as 700-1,000 kWh/ton lead, and environmental pollution and equipment corrosion resulted from the fluorine-containing solution. A new H2—PbO fuel cell process reported by a research group led by Panjunqing eliminates the demand for electrolysis in the existing hydrometallurgy process and absorbs the advantages of fuel cell and redox flow cell. In that process, PbO is dissolved in alkaline NaOH solution, high-purity Pb is recovered in the form of H2—PbO self-power generation, and the energy consumption and electrolysis cost in the lead recovery process are greatly reduced, and the lead recovery cost is lower than the cost of existing pyrometallurgy process (Nature Communications, 2013, 4, 2178:1-6). Though it is expected that the cost of future industrial hydrometallurgy process of lead recovery will be lower than the cost of lead pyrometallurgy, we are still considering whether the existing lead recovery concept is appropriate. Through analysis of the entire history from the first time of lead smelting thousands of years ago to the modern H2—PbO fuel cell process, it is found that the metallic lead concept for lead recovery has been followed in the large-scale lead recovery field; in contrast, the main modern lead customers have turned from conventional lead letter casting, lead cables, acid-proof lead storage tanks, and lead-acid batteries to the lead-acid battery market increasingly. For lead-acid battery manufacturers, the active material in lead-acid batteries is lead oxide, and only some refined lead is required to produce alloy plate grids (e.g. Pb—Ca plate grids). Hence, while lead smelting enterprises consume huge energy to smelt lead-containing materials (e.g., lead oxide) into crude lead and then electrolyze the crude lead into refined lead, their major customers—lead-acid battery manufacturers buy refined leads, melt the refined lead and cast into lead balls, and finally oxidize the lead balls into lead oxide by ball milling and use the lead oxide as an active material in lead-acid batteries. It can be seen that the lead smelting enterprises have produced a large quantity of refined lead according to the traditional concept and accordingly result in huge energy consumption and severe environmental pollution in that process, without taking consideration of the actual demand of their main customers—lead-acid battery manufacturers. Hence, the conventional lead pyrometallurgy industry must change the traditional lead smelting concept that involves high energy consumption and severe pollution to a new concept of directly producing lead oxide. For waste and worn lead-acid batteries, how to seek for an effective method to effectively convert the four components (Pb, PbO, PbSO4, and PbO2) in waste lead paste into pure PbO is a difficult task in the lead oxide regeneration process. As disclosed in existing patent literatures, some research groups have tried to prepare lead oxide from waste lead paste. For example, in CN103374657A, a raw material (e.g., sodium carbonate) and waste lead paste have a desulphurization reaction, then the desulphurized lead paste has a reaction with citric acid solution; next, through filtering, washing, and drying procedures, lead citrate is obtained; finally, the lead citrate is calcined to obtain super-fine lead oxide. Though the target product in that invention is PbO, raw chemical materials such as citric acid, hydrogen peroxide, and sodium carbonate, etc. are consumed heavily. Therefore, that approach is uneconomical when viewed from the aspect of atom utilization; in addition, that process can't separate impurities (e.g., barium sulfate) originally included in the waste lead paste. In CN103374658A, a method for preparing super-fine lead oxide from desulphurized lead paste through a three-stage process is disclosed, comprising: step (1): dissolving lead paste that has been desulphurized with sodium carbonate and pre-reduced with hydrogen peroxide in nitric acid or acetic acid; step (2) controlling the acidic lead-containing solution to have a reaction with sodium carbonate to obtain lead carbonate; step (3): producing super-fine lead oxide that contains PbO, Pb3O4, or a mixture of them from lead carbonate by calcination. Apparently, the process is mainly a conventional process that consumes raw chemical materials. A large quantity of raw chemical materials, including hydrogen peroxide, nitric acid, and sodium carbonate, etc. are consumed in the lead recovery process.
Similarly, in CN102820496A, a process is disclosed. In that process, the lead paste obtained from waste lead-acid batteries reacts with acetic acid and H2O2 under a stirring condition, and then the mixture is filtered to obtain lead acetate crystals. Finally, the lead acetate crystals are calcined at a high temperature for 2-3 h, to obtain PbO powder.
As described above, existing lead oxide recovery processes reported up to now mainly comprise the following three procedures: (1) lead paste pre-reduction and pre-desulphurization; (2) convert the pretreated lead paste into a lead salt such as lead acetate or lead citrate, etc. by means of acetic acid, citric acid, or oxalic acid; (3) obtain lead oxide from the lead salt (lead acetate or lead citrate, etc.) by calcination. Since the target product is PbO, a green lead recovery process should include two parts: first, though the lead sulfate part has to be recovered with a desulfurizing agent, the recovery of other parts (Pb, PbO and PbO2) should not involve addition of any other atom as far as possible; second, an effective lead oxide purification process that is based on an atom-economic approach should be provided.
The research group led by Pan Junqing has made further research for improving economic atom utilization in the lead conversion process, and has disclosed a novel method for utilizing the lead paste in lead-acid batteries in CN103146923A. That method comprises the following five procedures: 1. heating the lead paste in lead-acid battery and lead powder to have a solid-phase mixing reaction; 2. carrying out alkaline desulphurization in NaOH solution A; 3. leaching the desulphurized product with NaOH solution B, to obtain lead-containing alkaline solution and filter residue, and then treating by purification and cooling crystallization to obtain lead oxide; 4. utilizing NaOH solution C to carry out recrystallization to obtain PbO crystals at a higher purity; 5. after desulphurization, adding NaOH in the NaOH solution A to precipitate sodium sulfate crystals; in that approach, a NaOH desulphurization cycle is created, with sodium sulfate as a byproduct. The features of that method include: for the four components of lead paste, firstly, Pb and PbO2 are utilized to directly obtain PbO in solid state, and the excessive PbO2 in the waste lead paste is consumed by adding Pb; secondly, only the PbSO4 in the lead paste is desulphurized to generate PbO and Na2SO4; finally, NaOH solution is utilized to control the PbO to conduct recrystallization, and thereby purer PbO solid is obtained. That method utilizes an atom-economic reaction between Pb and PbO2 and purifies PbO by recrystallization in NaOH solution. The raw material NaOH, which is mainly consumed, is only used for desulphurization of the PbSO4 in the lead paste. Thus, unlike other processes in which all components in the lead paste are converted into lead salt and then desulphurized, the process disclosed in that patent document exploits a novel lead oxide recovery technique from the aspect of improving economic atom utilization. Through research that lasted almost one year, the main drawbacks of that method become increasingly apparent, mainly including:
1. Long process flow: 5 procedures, including solid-phase reaction at a high temperature, desulphurization with NaOH solution A, leaching with NaOH solution B, recrystallization with NaOH solution C, and NaOH addition for sodium sulfate precipitation, are required. Therefore, it is very necessary to simplify the process and thereby reduce the recovery cost and energy consumption.2. PbSO4 doesn't participate in the reaction before/after heating, in the high-temperature solid-phase conversion of the lead paste in the first stage. Hence, the PbSO4, which accounts for 30-45 wt % of the total weight of the lead paste, is mingled with Pb and PbO2 and is heated up meaninglessly, resulting in energy waste; in addition, a great deal of lead sulfate included in the lead paste results in incomplete solid-phase reaction between Pb and PbO2, and consequently a part of unreacted Pb or PbO2 particles remain in the product. Hence, in order to improve product quality and recovery rate, it is of particular importance to eliminate the adverse effect of PbSO4 ahead of time.